@Randa Zoqlam has PM'd me this question and I think to others too.
Why is the intensity size very different from the number size?
Dear all, I'm developing positively charged PLGA nanoparticles and from the dls there is a significant difference between the Z-avg and the number. I know that the Z-avg is expected to be higher but I'm just wandering if it's expected to be that different ? sometimes I get intensity around 130 nm and the number around 27nm
Searching on ResearchGate and on Materials Talks will provide you the answer to this question. For example:
https://www.materials-talks.com/blog/2017/01/23/intensity-volume-number-which-size-is-correct/
https://www.researchgate.net/post/DLS_results_give_an_intensity_distribution_a_volume_distribution_and_a_number_distribution_Which_of_these_is_more_important_and_why
https://www.researchgate.net/post/Could_using_Dynamic_Light_Scattering_technique_for_nanoparticle_size_measurements
Small amounts of aggregates (which may be a show stopper in therapeutic protein applications) will be seen by a technique sensitive to the large material. DLS has a r6 or volume2 relationship to the scattered intensity and, as such, is extremely sensitive to a small amount of larger material including dust contamination. In a number-based distribution (which 'feels' right if we look under the microscope) then every particle is considered equivalent. If we consider value terms then 1 million 1 micron particles has the same value (= volume/mass) as a single 100 micron particle. The one in a million particle makes up 1/2 of the total mass (or value) of the system. It is truly the gold nugget in the gold mine.
We note from international standards (ISO22412:2017; ASTME2490-09(15)) that conversion from intensity to number is ill-advised. Indeed the term used in ISO is 'deprecated' which is pretty strong.
Intensity means the carrier of information in inverse space with respect to scattering & diffraction experiments where as numbers never goes like that.
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